The invention relates to a pressure sensor which is subjected to an external pressure, having a semiconductor chip which has a pressure-sensitive area, and a process for producing such a pressure sensor.
Such pressure sensors constructed on the basis of semiconductor materials and using what is known as the MEM technique or microelectromechanical technique are used for automotive applications, such as motor vehicle tires, and exhibit a high failure rate with increased requirements in relation to increased attack of aggressive media and increased accelerative loadings.
It is an object of the invention to provide a pressure sensor which withstands the increased requirements in relation to environmental influences, to accelerative and external pressure loadings and attacks of aggressive media.
According to the invention, the pressure sensor has a semiconductor chip having a pressure-sensitive area which is subjected to an external pressure, and contact areas which are arranged on a pressure-insensitive area of the semiconductor chip. The semiconductor chip is arranged in a hollow housing which has a housing base on which the semiconductor chip is adhesively bonded. The hollow housing additionally has a housing wall which surrounds the semiconductor chip and through which flat conductors project with an internal section into a housing interior. External sections of the flat conductors project out of the housing wall. Arranged between the contact areas of the semiconductor chip and the internal sections of the flat conductors are connecting elements which connect predetermined contact areas electrically to corresponding internal sections.
The connecting elements, the internal sections and the contact areas within the surrounding housing wall are covered by a first plastic compound. The pressure-sensitive area of the semiconductor chip is at least partly covered by a second plastic compound. Because of its material characteristics, under the same external pressure loading, the first plastic compound is subjected to lower deformations than the second plastic compound.
This pressure sensor according to the invention has the advantage that, as a result of extensive covering of the. pressure-insensitive areas within the housing of a first plastic compound with a negligible deformation, the deformation loadings at increased external pressure are reduced, in particular for deformation-sensitive components such as the connecting elements. As a result of limiting the highly deforming second plastic compound to a pressure-sensitive area of the sensor, the remaining areas and components within the housing are protected against distortions and displacements as a result of tensile, compressive and shear stresses during deformation of the pressure-sensitive area.
The pressure sensor according to the invention is able to satisfy the increased requirements on temperature-cycle resistance and pressure resistance and erosion resistance with respect to aggressive media without failing. A pressure sensor protected in this way by two different plastic components can advantageously be used for continuous operational monitoring of the tire pressure in rotating vehicle tires up to a tire pressure of 100 MPa without it being possible to determine large temperature hysteresis values in the operating temperature range between xe2x88x9250xc2x0 C. and +150xc2x0 C. The scatter in the temperature response is likewise reduced as compared with pressure sensors merely having a silicone gel covering on all sides.
The first plastic compound preferably has a thermosetting plastic made of an epoxy resin or a silicone resin. These resins, with appropriate fillers, can exhibit a coefficient of thermal expansion which is matched to the coefficient of expansion of the semiconductor material and/or the material of the hollow housing. The hollow housing has either a ceramic substance or a plastic material. In the ceramic substance or the plastic material, a transition layer of a flat conductor is embedded in such a way that an internal section of the flat conductor projects into the interior of the hollow housing and an outer section of the flat conductor projects outward from the housing wall.
The flat conductor is anchored in the hollow housing by the transition section. In order to support the internal section, the hollow housing can have a ledge on the housing inner wall, to which the internal section of the flat conductor is fitted. This ensures secure bonding of a bonding wire between a contact connection area of the internal section of the flat conductor and a bonding wire which is intended to connect the flat conductor to contact areas on the pressure-insensitive areas of the semiconductor chip.
In the event of thermal loading, in particular the flat connections between the bonding wire and the contact connecting area and the bonding wire and the contact area are endangered if, in these intrinsically pressure-insensitive areas of the pressure sensor, highly deformable plastic protective layers made of a resilient elastomer are applied. It is therefore advantageous to protect these areas not serving as sensors against thermal stresses and severe deformations of a covering plastic compound such as the second plastic compound. Furthermore, the first plastic compound adheres both to the pressure-insensitive areas of the semiconductor chip and to the inner walls of the hollow housing, so that the interfaces between the first plastic compound and semiconductor chip and also between the first plastic compound and the hollow housing are protected against aggressive media.
The second plastic compound preferably has a plastic gel of a resilient elastomer based on silicone. In this case, the high resilience permits protection of a membrane of semiconductor material arranged underneath in the pressure-sensitive area of the semiconductor chip without hysteresis effects building up. Such resilient elastomers are based on dimethyl polysiloxane or phenyl polysiloxane and can be used for operating temperatures in the range from xe2x88x9255xc2x0 C. to +200xc2x0 C. or xe2x88x92120xc2x0 C. to +200xc2x0 C., depending on the base material. A further preferred resilient elastomer is based on fluorosiloxane and can be used at operating temperatures between xe2x88x9255xc2x0 C. and +175xc2x0 C. Fluorosiloxanes of this type can be used in particular for the vehicle sector, since they are resistant with respect to fuels and solvents.
For resilient elastomers of this type, based on silicone, the energy loss factor at a predefined pressure cycle frequency, of the order of magnitude of minus four powers of ten, is extremely low, so that such a second plastic compound follows the deformations of the pressure-sensitive area of the semiconductor chip with negligible energy loss. Furthermore, elastomers based on silicone have the advantage that they can form intensive adhesion to silicone resins. The risk of microcracks in the interface between the first plastic compound and the second plastic compound can therefore be reduced if a silicone resin is used as the first plastic compound and an elastomer based on silicone is used as the second plastic compound.
The hollow housing can have a housing cover with an opening which leaves the pressure-sensitive area and the second plastic compound free. A housing cover of this type can advantageously be matched to the inner dimensions of the housing wall, by its external:dimensions permitting a clearance fit with respect to the inner dimensions of the housing wall of the hollow housing. Following the application of the first plastic compound and still before the crosslinking of the resin, this housing cover is pressed onto said resin, at the same time any joints with respect to the housing wall being sealed off. A second plastic compound can be introduced into the opening that leaves the pressure-sensitive area of the semiconductor chip free, before or else after the fitting of the housing cover.
In order to construct the semiconductor chip as a pressure sensor, the semiconductor chip has a hermetically sealed cavity under reference pressure. This cavity can have a cylindrical shape which is surrounded by a rigid semiconductor wall of semiconductor chip material. This cylindrical shape is sealed off on one side by a pressure-sensitive membrane of semiconductor chip material. This membrane of semiconductor chip material can thus form the pressure-sensitive area of the semiconductor chip. For this purpose, the membrane of semiconductor chip material can have at least one sputtered-on electrode a few nanometers thick, via which an electrical signal, which corresponds to the flexure of the membrane under pressure loading, can be generated.
The cavity in the semiconductor chip material is sealed off hermetically with respect to the housing base by means of a gas-tight adhesive layer between the housing base and the semiconductor wall. If, for example, a tire pressure is applied to the pressure sensor, the membrane of semiconductor chip material bulges inward, so that the distance between the electrode arranged on the membrane and an electrode fitted to the housing base decreases. In this way, for example, the resonant frequency of an RC tuned circuit or an LC tuned circuit may be shifted, so that the frequency shift represents a measure of the flexure of the membrane and therefore a measure of the external pressure with respect to the reference pressure in the cavity. A pressure sensor of this type according to the invention has the advantage that, on account of the two different plastic covering compounds, it is protected against aggressive media and, secondly, on account of the resilience of the semiconductor chip material, it is able to measure large external pressure changes without being damaged and with a negligible energy or attenuation loss.
In order to be able to apply supply voltages and supply currents to the contact areas of the semiconductor chip and to be able to pick up pressure-specific electrical signals from the pressure sensor, the flat conductors have external sections projecting out of the housing wall of the hollow housing. These external sections of the flat conductors can be arranged at the level of an outer underside of the base or at the level of the inner upper side of the housing base. An arrangement at the level of the inner housing base ensures that the underside of the housing base consists entirely of hollow housing material, as a result of which the flat conductors are embedded in a better protected and better anchored manner in the housing wall of the hollow housing. The anchoring of the flat conductors with their transition sections in the housing wall can be improved further if the flat conductors do not project rectilinearly through the housing wall but if the flat conductors additionally have a Z-shaped angled section within the housing wall.
The pressure sensor according to the invention can withstand external pressure loadings such as occur in vehicle tires and, furthermore, can withstand without damage extreme accelerations such as occur during the rotation of vehicle tires. The pressure sensor according to the invention is therefore suitable to be arranged as a permanent pressure sensor in the rotating vehicle tire. Furthermore, the pressure sensor can be used in motor vehicles at all locations which, firstly, are subjected to high mechanical loadings and, secondly, are subjected to environmental influences, in particular aggressive media.
A pressure sensor can be produced by the following process steps.
First of all, a hollow housing is provided, specifically with an incorporated semiconductor chip which has a pressure-sensitive area and pressure-insensitive areas. The hollow housing also has an opening which leaves at least the pressure-sensitive area free. The housing and the semiconductor chip are already connected to one another via corresponding electric connecting elements in such a way that external sections of flat conductors have access to the electrodes of the semiconductor chip. The hollow housing with semiconductor chip and connecting elements is then covered by a first plastic compound, while sealing the surfaces of housing inner walls and also surfaces of the pressure-insensitive areas of the semiconductor chip. During this application of a first plastic compound, the pressure-sensitive area of the semiconductor chip is left substantially free. A second plastic compound is then applied to the pressure-sensitive area of the semiconductor chip, while sealing the interfaces between the first and second plastic compound in a gas-tight manner.
Owning to its material characteristics and owing to its geometrical construction, the first plastic compound differs from the second plastic compound in that, given identical external pressure conditions, lower deformations occur in the first plastic compound. This process has the advantage that a pressure sensor is formed which has a covering made of two different plastic components, which differ fundamentally in their deformation behavior. Thus, deformations which occur in the pressure-sensitive area are not transmitted to the deformation-sensitive connecting elements.
Both the application of a first plastic compound and the application of a second plastic compound can be carried out using a simple dispensing technique. On the other hand, it is possible by means of molding to apply the first plastic compound first of all, which can be made of a deformation-resistant thermosetting plastic, and then to apply the resiliently deformable second plastic compound to the pressure-sensitive areas of the semiconductor chip by means of dispensing, spinning on or varnishing on. These techniques can also be carried out in an extremely inexpensive and cost-effective manner, so that the process costs remain low.
A large number of semiconductor chips with pressure-sensitive areas, such as are required for incorporation in a hollow housing, can be produced simultaneously and in parallel in the following manner.
First of all, semiconductor chip positions are defined on a semiconductor wafer. Then, a plurality of cavities are etched in at the semiconductor chip positions from the rear side of the semiconductor wafer. This wet-chemical etching by means of alkalis or acids or dry etching by means of a reactive plasma is continued until a translucent and/or pressure-sensitive membrane remains on the upper side of the semiconductor chip, in the semiconductor chip positions.
Then, electrodes can be applied selectively to the pressure-sensitive membranes on the semiconductor wafer, that is to say on its upper side. Electrodes of this type can be structured as a capacitor plate or as a measuring strip or as a filter pattern as an electrode of a travelling wave amplifier. Contact areas, which are connected to the electrodes via conductor tracks, are then applied to the pressure-insensitive upper side of the semiconductor wafer, that is to say in the areas in which there is no membrane. In addition, passive and active semiconductor components relating to integrated circuits and evaluation structures can already be introduced into the pressure-insensitive areas of the semiconductor chip. Finally, the semiconductor wafer is divided up into individual semiconductor chips having a cavity and a pressure-sensitive membrane and also contact areas.
One advantage of this process is that, for a plurality of semiconductor chips, both the cavities for a reference pressure and also sensor electrodes, contact areas and control and evaluation circuits are produced in parallel and simultaneously on a semiconductor wafer.
In parallel with the production of suitable semiconductor chips, hollow housings with a housing base, including the embedding of transition sections of flat conductors in housing walls, can be pressure die-cast or pressure-pressed on a flat lead frame. Pressure die-casting is used when hollow plastic housings are to be produced, while pressure pressing with subsequent sintering is preferred for hollow ceramic housings. A flat lead frame of this type can have a plurality of hollow housings one behind another on a flat conductor strip and in rows beside one another at appropriate component positions.
In an automatic fitting machine, the semiconductor chips can then be bonded with their cavities onto the housing bases of the hollow housings, sealing off the cavities in a gas-tight manner. After fitting, the flat lead frame having a plurality of hollow housings, which now have the semiconductor chips, can be put into a bonding machine, in which internal sections of the flat conductors are connected electrically to contact areas of the semiconductor chip via bonding wires. A process of this type is also -suitable for the mass production of pressure sensors, so that cost-effective production becomes possible.
Finally, as mentioned above, the first plastic compound is already applied, in which the connecting elements are embedded. The uncovered pressure-sensitive area of the semiconductor chip is then covered by the second plastic compound, as mentioned above. Finally, a further housing cover can be fitted to the hollow housing, leaving the pressure-sensitive area free and leaving the second plastic compound free.
In summary, it should be recorded that a pressure sensor according to the invention with an MEM structure (micro electro mechanical structure) has a pressure-transmitting layer in the form of a gel over the MEM structure, and this pressure-transmitting layer of gel can be reduced to a surface minimum without exerting stresses on the remaining part of the semiconductor chip, if the semiconductor chip is covered in a low-stress manner by means of two different processes and two different materials. For this purpose, in a trough-like body of a hollow housing, contact is made with the semiconductor chip and, by means of these two different processes, two different materials are applied. A pressure sensor of this type with MEM structure exhibits the following advantages in the functional tests:
low-stress encapsulation with improved accuracy of the MEM output,
improved adhesion between the encapsulation materials used, so that for the first time a required media compatibility for a tire pressure sensor is achieved,
for the first time, fulfilment of the required mechanical acceleration tests for such a tire pressure sensor,
optimization of the plastic compounds to the MEM structure of the semiconductor chip.
In essence, the invention comprises a combination of globtop around the semiconductor chip and also on the pressure-insensitive surface of the semiconductor chip, and a pressure-transmitting silicone gel in the smallest possible amount on the pressure-sensitive area of the semiconductor chip.